A simple agar cloning system has been developed in this laboratory which supports colony formation of up to 3000 B-lymphocytes by cells from normal mouse lymphoid organs. Apart from being a unique method for determining the proliferative capacity of individual lymphoid cells, the technique permits a variety of functional studies on the clonal progeny of individual cells and a wide range of studies to identify and characterise regulatory factors (including antigens) affecting this capacity for proliferation and differentiation. It is proposed to carry out studies on the origin of B-lymphocyte colony-forming cells (BL-CFC) in the fetal mouse and in organ cultures of fetal liver and spleen. This study will be extended into a clonal analysis of the development of BL-CFC in spleen colonies generated in adult irradiated mice by fetal or adult hemopoietic stem cells. In this study of individual spleen colonies a parallel agar culture analysis will be made of the development of progenitors of neutrophils, macrophages, eosinophils and megakaryocytes. Factors influencing the appearance of BL-CFC such as the number and source of stem cells, germ-free status and antigenic stimulation will also be investigated. Further analysis will be made of the factors operating in vitro to modify B-lymphocyte colony formation including the active factor released by macrophages. An analysis will be made of sera from normal and athymic mice after irradiation, antigenic stimulation or endotoxin injection in a search for humoral factors capable of being detected by the B-lymphocyte assay system. Finally further studies will be made on the clonal nature of B-lymphocyte colonies and their capacity to synthesize functionally active IgM and IgG antibodies.